This invention relates to an electrolytic process for the preparation of ammonia metatungstate from ammonium paratungstate.
A characteristic feature of the tungstates is the formation of condensed complex anions of isopolytungstates in acid solution. Isopolytungstates formed by the addition of hydrated acid (H.sub.3 O+) to the tungstate anion (WO.sub.4 --), have various ratios of the hydrated acid group to the tungstate anion. As the ratio of the hydrated acid groups to tungstate ion increases, the molecular weight increases and the pH is lowered. Paratungstates, metatungstates, and tungstic acid are formed at increasingly higher ratios.
These isopolytungstate compounds are widely used in a catalyst industry, usually as the ammonium salts, since the alkali metal and other salts would normally act as contaminates of the catalyst material.
Ammonium metatungstate may be prepared by a variety of techniques. According to one method, hydrated tungsten trioxide may be dissolved in ammonium tungstate solution until a pH of about 2.5 to 4.0 is reached. This technique requires the addition of large amounts of hydrated tungsten trioxide which becomes less soluble in solution with lower pH values. Filtering the excess may be difficult due to the tendency of the hydrated tungsten trioxide to form a colloid. Yields according to this method approximate 50 percent.
Another technique for the production of ammonium metatungstate is by addition of an ion exchange resin to ammonium tungstate solution or a slurry of ammonium paratungstate crystals until the required pH for formation of the ammonium metatungstate is achieved. The use of ion exchange resins requires regeneration of these materials by the use of a strong mineral acid and a consequent formation of unwanted salt by-products.
Another technique is described in U.S. Pat. No. 3,936,362 to Vanderpool et al. A tungsten compound may be produced by applying an electrical potential across an ion exchange membrane. The type of membrane utilized has anion exchange groups distributed throughout a polymeric matrix. Membranes which may be desirably used in this process are often fragile and not suitable for production of commercial quantities. Due to breakage or plugging of the membrane, it is necessary to replace or clean the membranes resulting in disruption of a commercial process.